The comments on this post are correct: It's relevant if and only if your paths will cross sufficiently that it could open the company up to claims of unfair procedures. If he is your direct or indirect report, if you would potentially be responsible for some form of disciplining him for his decisions in the workplace, or if you would be involved in determining whether or not he got the job-- all of those could result in him suing for unfair practices at the company. It doesn't mean that he'd win any of those suits-- it would be notoriously difficult to prove that you acted unprofessionally unless there was written documentation-- but it would look bad for you and the company if the relationship came up and you had not come forward about it. Most likely, it would have a negative impact on your career if there was some way that it was relevant and you had not come forward.
My personal opinion is that it's a better idea to disclose it to your boss, in writing, and make it their problem. Now someone else has to worry about ensuring that you're not placed in a potentially compromising position relative to your ex. This is also useful in case somewhere down the line, one of you shifts departments/earns a promotion, and you find yourselves in one of those professionally compromising situations, or if the ex behaves inappropriately towards you and you need to file for disciplinary action against them. I think you can remain professional by telling your boss something like, "I think it's in everyone's best interest for me to convey to you that I was previously romantically involved with this person prior to his employment here. We're not currently involved, and I have no interest in getting involved again. I just wish to share this information so that we can avoid a potentially litigious situation in the future where that relationship might open us up to claims of unfair treatment based on the relationship. I would appreciate you exercising discretion in sharing this information with other team members."
That being said-- this is your personal life, and you have a reasonable right to privacy. You did nothing wrong by having a relationship with this person, but it is reasonable and understandable if you do not wish to convey this information when it is not presently relevant, and may never be relevant. Should things change and you are in a position where you need to reveal your past relationship, you'll probably be asked why you didn't say anything earlier. It's reasonable to say that it wasn't relevant earlier, and you wished to keep your private life and your work life separate.
Because human beings are irrational creatures, there's a chance that this could have negative consequences for you. You'll have to use your own best judgement about how your higher-ups would react to you telling them something like this later on. Certainly some offices will have your back and agree with you that you had no obligation to share your personal life. Unfortunately, I know some of the crummier places I've worked in the past would apply unearned and unflattering social stigmas for it, even though they would have no merit. Most likely those insinuations would be verbal, and HR would just say "I've written them a strongly worded letter saying that they should not do that" as those unscrupulous individuals continue to harass you and damage your reputation. Obviously that could affect your quality of life at work and your reputation within the company.
